Process for producing semiconductive crystals of uniform resistivity



1956 E. BUEHLER ET AL 2,768,914

PROCESS FOR PRODUCING SEMICONDUCTIVE CRYSTALS OF UNIFORM RESISTIVITYFiled June 29, 1951 4 Sheets-Sheet 1 FIG.

G. K. TEAL MOSQW INVENTORS: HL ER Oct. 30, 1956 E. BUEHLER ET AL 72,768,914

PROCESS FOR PRODUCING SEMICONDUCTIVE 1 CRYSTALS OF UNIFORM RESISTIVITYFiled June 29. 1951 4 Sheets-Sheet 2 FIG. 2

- EBUEHLER IIYVENTORS- am TEAL A 7' TORNE V E. BUEHLER ET AL PROCESS FORPRODUCING SEMICONDUCTIVE Oct. 30, 1956 2,768,914

CRYSTALS OF UNIFORM RESISTIVITY Fiied June 29, 1951 4 Sheets-Sheet 5o/sr/wca -/NC/-/ES DISTANCE -/NC/1'E.5

LENGTH-INCHES F/G. 7A

INVENT-ORS; E1?

By ax. TEAL low 8.0m

ATTORNEY Oct. 30, 1956 E. B'JEHLER ET AL PROCESS FOR PRODUCINGSEMICONDUCTIVE CRYSTALS OF UNIFORM RESISTIVITY Filed June 29, 1951RESIST/WT) OHM CM 4 Sheets-Sheet 4 FIG. 8

0 I I I 0/5 TA/VCE- INCHES E. BUEHLER INVENTORS: a K TEAL ATTORNEYUnited States Patent O PROCESS FOR PRODUCING SEMICONDUCTWE CRYSTALS OFUNIFORM RESISTIVITY Ernest Buehler, Chatham Township, Morris County, and

Gordon K. Teal, Summit, N. J., assignors to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJune 29, 1951, Serial No. 234,408

Claims. (Cl. 1481.6)

This invention relates to methods of forming semiconductive materialsusable, for example, in transistor varying cross-sections, may bemulticrystalline or may consist of but a single crystal, may be ofconstant resistivity and excess impurity concentration at any desiredlevel over substantial portions of their length, may contain P-Nboundaries at any desired location or locations longitudinally orlaterally and of any desired electrical characteristics, and may containN-P-N junctions at any desired location or locations, and of tailoredelectrical specifications.

In short, the process of the present invention consists of the drawingof a crystal or crystals from a molten body of a semiconductive materialby dipping a seed crystal into the melt, allowing the interface of thecrystal and the melt to arrive at thermal equilibrium and pulling thecrystal at such a rate that the molten material crystallizes out on theseed. The forming crystalline body is rotated during the pullingprocedure so as to result in a mass with a symmetrical cross-sectionand'the pulling rate and melt temperature are constantly varied so as tocontrol the level of excess impurity present at any point in the formingmass. Also described are means of controlling resistivity andconductivity type additions of significant impurities and by heattreatment all during the drawing procedure. 7

It has previously been suggested that crystals of germanium could begrown from the melt using acons'tant pulling rate. It has been observed,however, that with a predominance of most significant impurities ingermanium, resistivity in the solid decreases progressively in such aprocess as the molten mass crystallizes, thus indicating a I of lessthan 1 for these impurities (I=impurity concentration in the solid overimpurity concentration in the liquid). That is to say, the formingcrystalline matter approximates the normal freezing curve of the alloyof germanium and whatever impurity is being used. An example of such afreezing curve for antimony in germanium may be found in The PhysicalReview,

vol.*-77, pages 809m 813, March 15, 1950,;lg5arson',

Struthers, Theuerer, Fig. 3.

By thefprocess of the present invention, the'normal freezing curve isavoided by varying'the' rate of pull. The generalprinciple, 'whereit isdesiredf to produce a crystal having a constant resistivity zoneand'where I is lessthan 1- (as it is in all impurities thus far.testeclfor which I has been computed); is to decrease the rate of pullgradually as the crystal is formed so as to entrap less and lessimpurity and so as to negate" the'naturaI tendency of the formingcrystal to follow its freezing curve. i .w

Were rate of pull to be the only variable in the process, the crystalwould increase iriacross-section.as draw'nifrom the. m t duet t drppinsirate pf. pull. lTolcounteract ,7

this effect and to maintain a constant cross-section over 2,768,914Patented Oct. 30, 1956 "ice 2 the constant resistivity Zone, thedecreasing pulling rates are balanced by an increasing melt temperature.The latter decreases the rate of crystallization and is monitored so asto be exactly equal and opposite in efiect to the increase incross-section that would result from the decreasing pulling rate. 4 v Ip p v The invention can be better understood by reference to theaccompanying drawings in which: p V

Fig. 1 is a front elevation of a piece of apparatus on which the processof this invention can be carried out;

Fig. 2 is a sectional view of the vibrating member which beats againstthe wire supporting the crystal in the apparatus of Fig. 1, therebyhaving a stirring effect; I

Fig. 3 is a front elevation of the crucible assembly embodied in theapparatus of Fig. l;

Fig. 4 is a front elevation, partly in section, of an alternate form ofcrucible assembly showing doping means whereby the resistivity andconductivity type of the forming crystal may be influenced by theaddition of controlled amounts of significant impurities;

Fig. 5 is a perspective view of still another form of crucible;

Figs. 6A and 6B are graphical representations of the variation ofresistivity along the length of specimens which may be produced underdifferent conditions of operation by the process of this invention;

Figs. 7A and 7B are graphical representations of the variation ofimpurity concentration along the length of specimens which may beproduced under different conditions by the process of this invention;

Fig. 8 is a graphical representation of resistivity variation withlength under still other conditions of operation.

Referring to Fig. 1, the apparatus is put into use as follows:

An ingot of the semiconductive material is placed in carbon crucible 1,a seed 2 of the same material 1s placed in chuck. 3; crucible assembly 4on which is mounted crucible 1 is then inserted in the bottom of quartzenvelope 5. The equipment is flushed by passing. nitrogen into gas inlet6, through envelope 5 and gas outlet 7. After the system has beenflushed with nitrogen, hydrogen or some other gas which will have aminimum effect on the composition of the forming crystal is then allowedto, circulate by the same route and is continued throughout the process.A high frequency generator, not shown, is then turned on and a currentis passed through induction coil 8 in order to heat carbon crucible 1.After the ingot is completely molten, spindle 9 to which seed 2 isattached is then lowered until the seed just touches the melt.Vibratorlt) and rotator 11 are then turned on.

After the desired waiting period, motor 12 is turned on If what isdesired is a single crystal of germanium,-the

process is allowed to.-proceed until all the melt has-been withdrawnwith appropriate variation in pulling rateand in temperature 'of themelt as later described. If it is desired to form aP-N boundary bygasdoping at the appropriate. point in the process, valves 16 and 17 areopened and valve 118is closed thus allowing hydrogen to pass throughinlet 19'through the reservoir containing the desired doping substancein liquid form 20 and gaseous form 21, thus sweeping doping gas 21through flow meter 22 into atmosphere of quartz envelope 5. This dopinggas could alternatively be passed directly into the melt through base 4,through a hole in the crucible not shown, or through doping tube 23. i I

v In certain cases it is desirable to; dope by meansvof solid pellet.Where this-.is desired, the desired number and sequence of pellets maybe mounted in magazine 24.

At the desired time, motor 25 is turned on. This motor is directlycoupled to dispenser 26 containing an opening which is brought in linewith one chamber of magazine 24 thereby allowing a pellet to flowthrough tube 23 into the melt.

Throughout the process, cooling water is passed through water inlet 4Ainto a water jacket surrounding crucible 1 and is discharged throughwater outlet 4B. The upper portion of quartz tube 5 is also cooled bypassing water through 40 and out 4D. Switches A, B, and C control,respectively, rotation, vibration and solid doping. Valve D controls gasinlet 6.

Fig. 2 is a detail drawing of vibrating member 19. Eccentric 27 mountedon the shaft of motor 28 causes .shoe 29 to vibrate cable 14.

Fig. 3 is a detail view of the crucible assembly. From this figure, onemay get a clearer picture of base 4 and crucible 1. The temperatureinduced in the melt and crucible 1. by coil 8 is controlled by means ofan electrical circuit, not shown, monitored by thermocouple 30.

Fig. 4 illustrates an alternate crucible assembly construction to permitsolid pellet doping. By this process the pellet 31 is placed in hole 32and is supported by quartz rod 33 which can be raised above the surfaceof the bottom of the crucible by actuating steel rod 34. By thisalternate process, pellet 31 is melted along with the melt in thecrucible so that the doping material enters the melt not as a solidpellet but, rather, as a molten doping alloy.

Fig. 5 illustrates a crucible assembly for yet another alternate methodof doping. By this process the pellet 31 is placed in an indentation onthe rim of crucible 1. After this pellet has become molten and at thedesired time, actuating member 32 which may be made of quartz or carbonis rotated so as to push the molten doping substance over the rim of thecrucible 1 and into the melt.

Figs. 6A and 6B are plots of resistivity against distance measured bytwo point probes along single undoped crystals of germanium produced bythis process. As may be seen from curve 6A, the resistivity of theinitial portion of the crystal is dropped from point 35 to 36 while thecross-section of the crystal is allowed to build up. The portion of thecurve between points 36 and 37 represents the desired constantresistivity zone which is produced by monitoring the drawing rate andthe temperature as will be later described. After the drawing rate hascome to a virtual standstill at point 37 so that no more crystal ofresistivity 36-37 can be drawn, the resistivity is allowed to drop to avaluerepresented bypoint 38 by a sudden increase in drawing rate. Thedrawing rate and temperatures are again monitored so as to result inflatportion 38-39. After the drawing rate has again come to a virtualstandstill the rate of drawing is allowed to remain constant so as todraw up the remaining portion of the ingot resulting in normal freezingcurve Sid-4t Fig. 6B is" similar to Fig. 6A but represents a crystalhaving only one'flat resistivity zone. Increasing the pulling rateanddecreasing the temperature to maintain the cross-section resulted ingradient 41-42. Zone 4-2-43 resulted from a normal monitoring of pullingand temperature rates as above described. Portion 43-4 represents thenormal freezing curve in which the remainder of the ingot was drawn atany constant rate. I

1 Figs. 7A and 7B are graphical representations of crystals which havebeen doped. The coordinates are log of excess impurity concentrationexpressed in atoms per cubic centimeter against length expressed ininches. The

I log of excess impurity concentration in the negative or downwarddirection represents a p conductivity type of decreasing resistivity,while the positive portion of the vertical represents n conductivitytype. The resistivity values may be determined from the excess impurityconcentration values by use of the equation neg.

where p represents resistivity in ohm-centimeters, n equals excessimpurity concentration expressed in atoms per cubic centimeter, e is thecharge on the electron, while ,u is the mobility of the electrons orholes expressed in centimeters squared per volt second. Curve A of Fig.7A represents the crystal which has been doped either with one largepellet or one burst of gas as later described. Zone 45-46 of this figurerepresents the constant resistivity section produced by monitoring thepulling rate and temperature. The pelletis dropped into the melt atpoint 46 resulting in sudden change in conductivity type 46-47 whilesection 47-43 represents that part of the crystal drawn from theremaining portion of the ingot. Although this portion of the curveappears to be flatter than the corresponding portions of Fig. 6A or Fig.6B, both plotted in terms of resistivity against distance, it wouldappear similar to the normal freezing curve 39-40 if expressed on thosecoordinates.

Curve B in Fig. 7A represents a crystal with a P-N junction, produced bydoping as in curve A except that by means of a controllable dopingprocess using either multiple pellet or gradual gas doping thetransition range is spread out from point 51 to point 52. Zone 49-50again represents theconstant resistivity portion produced by the-normalmonitoring process as will be described. The addition of a first pelletor burst of gas results in zone 50-51; zone 52-53 results from a similaraddition of a pellet or burst of gas of opposite impurity type, while53-54 is the normal freezing portion of the crystal resulting fromdrawing the remaining portion of the melt. Curve C of this same figurerepresents a crystal formed either through constant gas doping ormultiple pellet doping resulting in a constant slope of curve 55-56.

Fig. 7B represents three illustrative crystals which have been dopedduring the drawing process so as to result in three different types ofN-P-N junctions. Portion 57-58 of curve A represents the constantresistivity zone brought about as otherwise described, while 58-59 isproduced by doping with a single pellet. Zone 60-61 represents dopingwith .a pellet of opposite semiconductivity type so as to bring thecrystal back into the 11 region. ioint 61 need not be higher than zone57-58 but in this specimen the crystal was brought back to a higher Itlevel at this point so as to produce a higher excess of n-type impuritywhich is desirable in a good emitter. Section 61-62 could again be anormal freezing portion of the crystal. The breadth d of the p region isdetermined by the time lapse between doping steps 58-59 and 60-61.

Curve B of Fig. 7B is similar to curve B of Fig. 7A except that afterthe crystal has been brought into the p region it is brought back intothe n region by a reversed doping procedure using a pellet or gas ofopposite semiconductivity type. Therefore, section 63-64 represents theconstant resistivity zone, while 64-65 represents doping with a firstpellet or burst of gas. Section 65-66 represents controlled dopingeither withpellets or with controlled bursts. of gas so as to producethe transition region. Section 66-67 represents doping with a largepellet-or with a burst of gas. Afteran interval represented by d whichmay be controlled by varying pulling rate or time sequence the melt isdoped with an impurity of opposite semiconductivity type'either bypellet or burst of gas to bring the crystal back to point 69. Point69-70 represents controlled doping either by pellet orby a burst of gas.Point 70-71 represents doping in this example,

of equal but opposite semiconductivity type to that of,

zone 64-65, while 71-72 could again be :normal freezing zone.

Curve C of Fig. 7B represents controlled gas dopingor multiple pelletdoping whereby the crystal is brought from point 73-74 either byconstant gas doping or mul tiple pellet doping. Point 74-75 representsthe region in which no impurity is added; This zone may -'or' may not beof constant resistivity type as desired. Zone 7576 is the reverse of73-74 and represents doping of an opposite semiconductivity type broughtabout either by gradual gas doping or multiple pellet doping.

Fig. 8 is a plot of resistivity against distance of a single crystal ofgermanium containing two constant resistivity zones 7879 and 80-81similar tothose of the crystal of Fig. 6A but containing two N-P-Njunctions 7980 and 81-82 which were produced thermally and without theaddition of any significant impurities by doping.

Although in theory, simply varying the pulling rates and varying thetemperature as above described should result in a smooth, circular rodof controlled electrical characteristics, in fact, this was proved notto be the case due to a thermal gradient across the surface of the meltand consequent varying rates of crystallization on different portions ofthe forming rod. Not only were the surfaces of the forming rod caused toform in irregular fashion, but the electrical characteristics of thegermanium were not kept constant even in cross-section. It was foundthat this could be overcome by rotating the crystal as it formed.Rotation rates of from 50 to 5,000 revolutions per minute aresatisfactory and a rate of several hundred, as for instance 200 to 500,is to be preferred.

Although it was found that so rotating a crystal had sufiicient stirringaction to result in a rod of circular cross-section, at first glance, itwas noted that ring-like irregularities were forming on the surface ofthe rod, especially at the lower speeds of rotation. This indicated thatthe temperature gradient, although diminished, was still in evidence.

It was found that this irregularity could be eliminated by the additionof a pumping action. This was done by adding a vibrating member placedin such a position so as to alternately stretch and contract the cableto which was connected the forming crystal. Vibration rates of frombeats per second at an amplitude of about 10 mils to about 500 beats persecond at-an amplitude of about of a mil have been found to besatisfactory, although here as with the stated rotation rates, thevalues cited are only suggestive and do not represent absolute limits.There is no apparent reason why they cannot be exceeded in eitherdirection. Both rates are limited on the low side by ineifectiveness andon the high side pri-' marily by equipment capability.

The crystals of the present invention may be formed from germaniumingots such as described in the copending application of J. H. Scaff andH. C. Theuerer, Serial No. 638,351, filed December 29, 1945, except thatin order to minimize impurity content, the last portion ofthe ingotformed is cropped once and is remelted, refrozen and recropped as oftenas is required depending on the purity desired. Varying this initialcharge will, of course, vary the electrical characteristics of the finacrystal.

. Pulling rates of from .0001 up to .006 inch per second are, habituallyused. At rates higher than .006 inch per second, and, under somecircumstances, even at this rate, a torsional strain results in thecrystal causing twinning, which is generally considered'undesirable insingle crystals. Fo'rzthis reason, it. ispreferred not to exceed about.003 inch per second. The lower value represents only a'pr'acticallimit. As will be seenfrom Example 9 below, it is sometimes desirable.to reduce the rate of pull to a complete standstill. a

As above set forth, when varying the pulling ratepit is'necessary alsoto vary the temperature of the melt where it is desired to. maintain thecross-sectional diameterv of the forming rod constant. A typicalschedule follows:

6 Although this monitoring may be done by hand, it has been founddesirable to use an automatic programming control, several types ofwhich are well known in the commercial chemical production field. Such adevice is adjusted so as to vary pulling rates and temperature inaccordance with the schedule above set forth. It should be noted thatthe tabulated temperatures represent measurements made at the wall ofthe crucible. Some time lag from the wall of the crucible to the melt isto be expected. 7

In order to maintain the lifetimes of the holes and conductivityelectrons in the forming rod at the highest possible level, it isdesirable to cool the rod as soon after it emerges as possible. This isdone by means of water cooling coils and a constant fiow of cooling gasthrough the envelope. Any good thermally conducting, non-oxidizing andotherwise non-reactive gas will be satisfactory for this purpose.Hydrogen, helium and nitrogen have been found to be satisfactory in thiscase.

One of the most important features of the process of this invention isthe flexibility with which semiconductive transition zones may beproduced. This may be done in two ways. The most important consists ofdoping either with a solid or with a gas containing the desiredsignificant impurity in amount sufficient to carry the material over tothe opposite semiconductivity type. Solid doping of crystals formed fromcharges of about 50 grams utilizes pellets of from 1 milligram up to 50milligrams of alloys of germanium together with the desired impurity, orof any compound of the impurity which will have the effect of injectingthe desired impurity into the melt.

The most satisfactory doping elements are gallium and boron for N to Pconversion and arsenic and antimony where P to N conversion is desired.For solid doping the elements may be added in elemental form or in theform of any alloy or compound which will result in the addition of theimpurity to the melt. Examples are theoxides and germanium alloys. Thetrichlorides have been found to be satisfactory in gas doping. Again theamounts of impurity used either in solid doping or in gas doping willvary with the size and purity of the ini' tial charge, the amount ofcharge left in the crucible and the results desired. As has been seen,pellet doping may be carried out with the pellets in solid form or inmolten form, and P-N or N-P-N junctions may be tailored to have anydesired characteristics by varying in solid doping by the size of thepellets, composition of pellets, and time sequence of addition of saidpellets; and in gas doping by the rate at which the gas is allowed tocome in contact with the melt, and on whether or not it is passed in inbursts or gradually. With either type of doping, the junctions may bevaried by, controlling pulling rates and the temperature of the melt.

Another method of producing P-N and N-P-N junctions without addition ofimpurities is by the simultaneous variation of temperature and pullingrate. Here it is necessary to choose a starting material such that heattreatment at about 980 C. will produce a swing of semiconductivity typefrom N to P. An example of such a material is a germanium of aresistivity of at least 10 ohm-centimeters.

It may be noted here that a twin boundary maybe formed eitherby usingtwo seed crystals side by side, or by starting with a seed containing atwin boundary;

Where it is desired to produce a maximum amount of .constant resistivitymaterial, and after, through monitoring, the rate of pull has beenreduced to a virtual standstill so as to make possible only a negligibleincrease increasing the pullingrate to some maximum value suddenly andthen monitoring its decrease as wasgdonein' the first zone. Z Sincethenormal freezing curve on co ordinates of resistivity against lengthis generally changing slope in this zone, it has been found preferableto decelerate the rate of pull more rapidly during the formation of thissecond portion of the crystal.

Although crystals may be grown using seeds of any crystallineorientation, it has been found preferable to either place the seed insuch a position or to grind it in such a manner as to approximate the[100] or the [111] orientation.

A general description of a typical process for the production of asingle crystal of germanium containing two constant resistivity zonesfollows: The seed crystal, cut from a rod produced by this process, isfirst cleaned and mounted in the chuck of the spindle. The charge, a onehundred gram germanium ingot such as produced by the process ofcopending application Serial No. 638.351. filed December 29, 1945, isloaded in the crucible. and the crucible assembly is mounted inposition. The induction coil is turned on and the charge is melted at atemperature of about 980 C. The seed is immersed in a melt to a depth ofabout mils and is left in this position sutliciently long to result inthermal equilibrium of the interface. A period of about five minutes hasbeen found to be satisfactory. With the seed crystal being pulled fromthe melt at an initial rate of about .003 inch per second, the rotatingmechanism is turned on and the vibrator is set into play. Thetemperature is dropped to about 935 C. and the pulling rate is keptconstant for a period of three minutes. During this three-minuteinterval, the diameter of the crystal has increased to about M; of aninch. Thereafter, where it is desirable to form a single crystal ofuniform diameter, the rate of pull and the temperature are programmed asfollows:

Temperature Increased to (Degrees Centigrade) Pulling Rate Decreased to(Inches per Second) The process thus far will result in approximately a1 /2 inch constant-resistivity zone of about ohm-centimeters. When thepulling rate has been reduced to a virtual standstill, the resistivityis dropped down to about half the level of the first zone by increasingthe pulling rate to .003 inch per second suddenly. During the formationof this second Zone, the pulling rates and temperatures are programmedexactly as above set forth, except at approximately double thedeceleration rate. The rate of pull is then increased to any value andallowed to remain constant until the remainder of the melt has beendrawn off.

If, at any time during the formation of the germanium crystal, it isdesirable tof orm P-N or N-P-N boundaries, this may be done by pellet orgas doping by any one of the methods heretofore described. Typicalexamples of P-N and N-P-N boundaries that may be tailcred tospecification by varying pellet size and/or frequency, by varying gasdoping rates, and/or by heat treatment, may be seen in Examples 3through 9.

Examples of how the above processes may be modified to produce ninedifierent types of crystals corresponding to the curves of Figs. 6A, 63,7A and 7B and 8 follow: Example 1 003 inch per second. melt temperaturewas dropped from an initial value of 980 C. to a value of about 935 (3.,the temperature change taking place in .a period of about ten seconds.The pulling rate was maintained constant at the rate of about .003 'inchper second for about three minutes during which time the diameter of theinitial portion of the crystal increased to about A; of an inch. Thepulling rate and temperature were then programmed as set forth in thegeneral description of the process above set forth. After the pullingrate had been reduced to a virtual standstill, the pulling rate was thenincreased suddenly to about .003 inch per second and once again thepulling rates and temperatures were programmed except at twice thedeceleration rate. The remaining portion of the melt was then allowed tobe drawn off at a rate of .003 inch per second. A crystal so produced ishere referred to as a two-step crystal. This particular specimencontained two constant resistivity zones, one of about 10ohm-centimeters running for a length of about an inch and a half, whilethe second zone had a resistivity of about 5 ohm-centimeters andextended for about A of an inch. Where it is desirable to producesemiconductive material of constant resistivity, and where it ispossible to use materials of both resistivity levels, it is thismodification of the invention which produces the highest degree ofefficiency.

Example 2 Temperature Drawing Rate (Inch-es per Second) (DegreesCentigrade) In the example shown in Fig. 6B, the pulling rate was variedas shown over a period of from two to three minutes. The pulling rateswere then decreased and the temperature was then increased, as scheduledin Example 1. This monitoring sequence took about fifteen minutes andresulted in a constant resistivity zone of about 3 /2 ohm-centimetersover a length of about 1% inches. The remainder of the melt was then runed at any desired speed resulting in the normal freezing portion 43-44of Fig. 6B. The rate of pulling used in this final portion has not beenspecified since to date this portion of the crystal has not found use inany transistor or rectifier device and is simply reused as part of theload of another run.

Example 3 The seed was mounted, the charge was loaded and the equipmentwas set into operation, as above described. The pulling rate was allowedto drop to about .0001 inch per second, while monitoring the temperatureand pulling rates as above scheduled. The pulling rate was allowed toremain constant at this value for about seven or eight minutes and allthe desired impurity was added at this point in the form of. a pellet.In order to obtain a crystal of electrical characteristics such asindicated by curve A of Fig. 7A, a lump sum of impurity which wouldresult in a total change in the crystal of about 10 atoms per cubiccentimeter was added. For an ingot of about 50 grams, this amounted to apellet of approximately 10 milligrams of gallium dioxide. An equivalentamount of boron in the form of a germanium alloy could have been.

used. This. could. alsohavebeen done with one blast of a doping gas suchas boron trichloride or gallium trichloride, containing approximatelytwice as much of the desired impurity, since only about half of the gasgoes into solution with the melt. The pulling rate was then raised toabout .001 inch per second and the remainder of the melt was drawn out.The materials produced by such a doping process are usable in voltageregulating devices due to their low back voltage.

Example 4 Multiple pellet doping or gradual gas doping was resorted toin order to obtain a crystal with characteristics corresponding to curveB of Fig. 7A. The process was put into operation exactly as set forth inExample 3, and after the pulling rate had been allowed to continue at.0001 inch per second for several minutes, one pellet consisting ofabout 2 milligrams of a gallium-germanium alloy containing about .35percent gallium was dropped into the melt by any of the doping methodsbefore described. This was done in order to bring the excessconcentration of impurity to about 10 atoms per cubic centimeters. Three2-milligram pellets of a gallium-germanium alloy containing about .05percent gallium, spaced about one second apart, were dropped into themelt. Finally, a second S-milligram pellet of galliumgermanium alloy,containing about 1.4 percent gallium, was dropped into the melt. Theremainder of the melt was then drawn out at a constant rate of about.001 inch per second. It should be noted that the breadth and electricalcharacteristics, such as the back voltage of the P-N boundary soproduced, may be tailored as desired by varying any one of thefollowing: the size of the pellet, the composition of the pellet, thenumber of pellets used, the time sequence and the drawing rate. It isalso possible to produce a crystal corresponding to this curve by usinggas doping. The first doping step could be by means of a burst of about10 cubic centimeters of hydrogen gas containing in the neighborhood ofX10 atoms per cubic centimeter of gallium trichloride or any other gascontaining the desired impurity of sufficient vapor pressure. The seconddoping step would consist of passing into the melt a doping gas at aconstant rate over about one minute and containing suflicient impuritysuch that the level of the forming crystal is varied a total of 5x10atoms per cubic centimeter. In order to bring the curve down to itsfinal resistivity level, another short burst of about cubic centimetersof hydrogen containing about the same amount of impurity is allowed tocome in contact with the melt. Crystals so produced contain excellentP-N junctions and may be tailored so as to meet the exactingspecifications of any desired rectifier or transistor structure.

Example 5 In order to produce a crystal corresponding with curve C ofFig. 7A, the process was set into motion as above described, except thatthe initial pulling rate was .001 inch per second. The diameter of thecrystal was built up to about M; of an inch by dropping the temperatureto about 935 C. whilethe pulling rate was maintained constant for fromseven'to eight minutes. Hydrogen containing suflicient impurity to varythe excess impurity level of the forming crystal about 5 X10 atoms percubic centimeter of the desirable impurity in any utilizable gaseousform was then added at a constant rate over a period of from three tofour minutes always keeping the pulling rate constant at about .001 inchper second. The only requirement for the gaseous compound here usedother than that it be of sufiicient vapor pressure is that it should notbe one which will impair the semiconductive properties of the crystalform and that it be non-corrosive to the apparatus used. As above setforth, the trichlorides of gallium, and boron are satisfactory. It isconceivable that such a crystal could be produced using a very largenumber of very small pellets, although as Curve A of Fig. 7B representsa doping process similar to that of curve A of Fig. 7A, except thatafter the semiconductive type was brought into the p region a pelletcontaining donor impurity was added. Such a run follows. After thepulling rate was maintained constant at .0001 for several minutes, andagain with a SO-gram starting charge, a S-milligram pellet of agallium-germanium alloy containing about 1.4 percent gallium was addedto the melt. After a time lapse of from five to ten seconds, al0-milligram pellet of arsenic trioxide or pure arsenic was added (thereason that an arsenic-germanium alloy was not used here is that thesolubility of arsenic in germanium is so slight as would make the alloypellet prohibitive in size). After the second doping step, the drawingrate of about .0001 inch. per second was maintained for about eightminutes. After this period the drawing rate was increased to about .001inch per second and the remainder of the melt was drawn out. Materialsformed by this process have been successfully used in N-P-N transistorstructures such as those described in patent application of W. Shockley,Serial No. 34,423, filed June 26, 1948. Varying the breadth of the keyregion represented by d on the curve will affect the frequency cut-offof the final transistor structure. The smaller at, the higher thefrequency cut-off. The breadth of this region may be varied by varyingthe time interval between the two pellets and/or by varying the drawingrate. The specimen of curve A of Fig. 7B was brought to a higherresistivity level after the key region, since this specimen was to beused as a transistor and since this zone was to be used as an emitter.An increased number of excess impurity atoms are desirable in that zoneof a transistor which is to be used as an emitter.

Example 7 The initial portion of the crystal represented by curve B ofFig. 7B was produced by the same doping amount and sequence as crystalrepresented by curve B of Fig. 7A. After the last dopingstep, however,the rate of pull was maintained constant at .0001 inch per second forabout five minutes, whereupon the same doping steps were repeated ininverse order using donor rather than acceptor impurity. Such materialsalso find use in N-P-N transistor structures.

Example 8 The crystal represented by curve C of Fig. 7B was produced byconstant gas doping or, alternately, multipellet doping at twice therate and half the time used in Example 5. After a time lapse of aboutfifteen minutes, the same gradual doping process was repeated, usingimpurity of opposite semiconductive type.

Example 9 To form a crystal corresponding to the curve of Fig. 8, it wasnecessary to start with the initial portion of the drawn rod at aresistivity level of at least 10 ohm-centimeters and preferably at least20 ohm-centimeters. After the process had been set into motion asdescribed in Example 1, and after the first constant resistivity zonehad been formed at a level of about 20 ohm-centimeters, the rate of pullwas reduced to zero, while the temperature was maintained at about 980C. for a period of several minutes, whereupon a second constantresistivity zone was formed exactly as in Example 1, the level of thesecond zone, in this instance, being in the neighborhood of 10ohm-centimeters. The pulling was again stopped, and the temperature wasallowed to remain constant at about 980 C. for several minutes,thereafter the remainder of the melt was drawn out at some constantrate.

Zone 7778 represents that portion of the crystal drawn at a constantrate of .003 inch per second, during which time the diameter was allowedto build up to some desired value (in this specimen 'V; of an inch).Zone '78-'79 represents a constant resistivity zone produced bydecreasing the rate of pull from .003 inch per second down to 0 inch persecond, and a simultaneous increase of the temperature of the melt offrom 935 C. to 980 C. Zone 7980, which in this specimen was about of aninch in thickness, resulted from keeping that portion of the crystal incontact with the melt for a period of about five to ten minutes,resulting in thermal conversion from N to P type. Zone Sit-81 representsa second constant resistivity zone formed in the same manner within thesame monitoring limits used in the formation of the first constantresistivity zone but at double the deceleration rate. 'P-zone 81-82 wasproduced by a second thermal conversion in the same manner and for thesame time as thatused in zone 7980, while 8233 represents that portionof the crystal which followed its normal freezing curve, resulting fromdrawing out the remainder of the melt at a constant pull rate of .001inch per second. Material containing such N-P-N transition zones isproduced without the use of any doping mechanism. There is, of course,no reason why this thermal conversion step may not be combined with anyone of the doping methods above described so as to give any desiredflexibility in the formation of the N-P-N transition areas. Crystals produced by the thermal conversion process above outlined have electricalcharacteristics making them suitable for use in devices such as thosedisclosed in the application of W. Shockley, Serial No. 34,423, filedJune 26, 1948, now abandoned.

Although most of the description is in terms of germanium it is to beunderstood that the described process works equally well with othersemiconductive materials. Silicon, for example, has been used and thecrystals produced from this material have been found to possessexcellent properties.

What is claimed is:

1. The method of producing a crystal of normally solid semiconductivematerial containing a significant impurity having a l of less than 1comprising inserting a seed into a melt of the said semiconductivematerial containing the said significant impurity, and withdrawing thesaid seed at a decreasing withdrawal rate such as to substantiallycompensate for the resulting increasing concentration of the saidsignificant impurity in the melt and such as to produce a crystalmanifesting substantially uniform resistivity over that portion of thecrystal produced While decreasing the withdrawal rate, the rate ofwithdrawal being at all times such as to maintain a solid-liquidinterface between the crystal and the melt, 1" being defined as theratio of impurity concentration in the solid to the impurityconcentration in the liquid with the two phases at equilibrium.

2. The process of claim 1 in which the temperature of the melt isprogressivel increased as the withdrawal rate is decreased in suchmanner as to maintain the crosssection of the crystal substantiallyconstant.

3. The process of claim 1 in which the crystal is 1'0- tated duringwithdrawal.

4-. The process of claim 3 in which the normally solid semiconductivematerial is germanium.

5. The process of claim 2 in which the normally .solid semiconducti'v'ematerial is silicon.

References Cited in the file of this patent UNITED STATES PATENTS1,353,571 Dreibrodt Sept. 21, 1920 1,531,784 Hazelett Mar. 31, 19251,921,934 Lewis Aug. 8, 1933 2,091,903 Baggett et al. Aug. 31, 19372,188,771 W'elch Jan. 30, 1940 2,514,879 Lark-Horowitz ct al. July 11,1950 2,530,110 Wcodyard Nov. 14, 1950 2,683,676 Little July 13, 1954FOREIGN PATENTS 706,858 Great Britain Apr. 7, 1954 OTHER REFERENCESJournal of Applied Physics, vol. 6, 1935, pages 111- 116.

1. THE METHOD OF PRODUCING A CRYSTAL OF NORMALLY SOLID SEMICONDUCTIVEMATERIAL CONTAINING A SIGNIFICANT IMPURITY HAVING A T OF LESS THAN 1COMPRISING INSERTING A SEED INTO A MELT OF THE SAID SEMICONDUCTIVEMATERIAL CONTAINING THE SAID SIGNIFICANT IMPURITY, AND WITHDRAWING THESAID SEED AT A DECREASING WITHDRAWAL RATE SUCH AS TO SUBSTANTIALLYCOMPENSATE FOR THE RESULTING INCREASING CONCENTRATION OF THE SAIDSIGNIFICANT IMPURITY IN THE MELT AND SUCH AS TO PRODUCE A CRYSTALMANIFESTING SUBSTANTIALLY UNIFORM RESISTIVITY OVER THAT PORTION OF THECRYSTAL PRODUCED WHILE DECREASING THE WITHDRAWAL RATE, THE RATE OFWITHDRAWAL BEING AT ALL TIMES SUCH AS TO MAINTAIN A SOLID-LIQUIDINTERFACE BETWEEN THE CRYSTAL AND THE MELT, T BEING DEFINED AS THE RATIOOF IMPURITY CONCENTRATION IN THE SOLID TO THE IMPURITY CONCENTRATION INTHE LIQUID WITH THE TWO PHASES AT EQUILIBRIUM.